1. Field of the Invention
The present invention relates to a reflecting liquid crystal display unit that performs display via reflection using liquid crystal.
2. Description of the Related Art
Various types of liquid crystal display units have conventionally been proposed. For example, a TFT liquid crystal device in which each pixel has a thin film transistor has been put into practical use. While a TFT liquid crystal device is capable of high precision display, it requires a precise and complex manufacturing process. Moreover, the manufacturing yield for such devices is poor, leading to an increase in costs.
With this as a backdrop, a polymer-based dispersed liquid crystal device, which is relatively easy to manufacture, has been drawing attention. This liquid crystal device uses a layer containing a composite of liquid crystal and polymer, which is created by dispersing a liquid crystal in a polymer material. For example, U.S. Pat. No. 5,437,811 discloses a liquid crystal display unit that performs color display by means of the selective reflection characteristic of a liquid crystal that exhibits a cholesteric characteristic. This liquid crystal device performs display via reflection using the focal conic state (a colored transparent state) and the planar state (a scattering state) of a chiral nematic liquid crystal. The focal conic state is a state in which the helical axes of liquid crystal molecules are aligned in a random fashion, whereas the planar state is a state in which the helical axes of liquid crystal molecules are aligned parallel to one another. This liquid crystal element scatters incident light in the focal conic state and selectively reflects light of a certain wavelength in the planar state. The liquid crystal device changes from the planar state to the focal conic state with the application of low voltage pulses, while it changes from the focal conic state to the planar state with the application of high voltage pulses.
Generally, the two states of said liquid crystal that exhibits a cholesteric characteristic have a stable memory capability. That is, even after the cease of voltage pulses, these states are maintained. Therefore, high precision display becomes possible using merely a simple matrix drive without the need for a complex circuit using active elements as in the case of TFT liquid crystal.
In the planar state, liquid crystal selectively reflects the light component having a specific wavelength among the rays of the incident light which enters the liquid crystal in parallel to the helical axes of the liquid crystal. The specific wavelength .lambda. corresponds to helical pitch p and average refractive index n of the liquid crystal, that is, .lambda.=n.times.p. The light that enters the liquid crystal parallel to the helical axes when the liquid crystal is in the planar state is divided into two types of circular polarization, i.e., right rotary polarization and left rotary polarization. The light component for one rotary direction passes through the liquid crystal while the light component for the other rotary direction is reflected completely by the liquid crystal. This property is called circular polarization dichroism. Because of this property, the reflection rate of the display unit in the planar state, i.e., the selective reflection state, is 50% at the maximum, and therefore where the planar state is used as a bistable display state, the luminance of the display is limited.
In order to resolve this problem, U.S. Pat. No. 5,408,344, for example, proposes a technology in which the luminance is improved by inserting a reflecting layer, located between the liquid crystal layer and the light absorbing layer (namely, opposite the observer side of the liquid crystal layer), having a scattering characteristic that varies depending on the angle of the incident light.
However, display units that use conventionally proposed liquid crystal-polymer composite layers have a problem that, where the illuminating light is fixed, the peak wavelength of the reflected light fluctuates greatly depending on the viewing angle, and the color of the display changes accordingly when observed. For example, if a reflecting liquid crystal display unit is placed on an indoor wall, e.g., a wall of a meeting room, light enters the liquid crystal display device at an angle from the illuminating source located on the ceiling. In such an environment, the display color changes depending on where the observer is situated.